Blender 2.8: Grease Pencil Fill Brush Not Working
The confusion is caused by the default setting of any new draw brush material where the fill is set to 0.0 alpha which means it is transparent (it makes sense as you'll most often just want to draw a stroke without a fill). Therefore the 'color bucket' click doesn't do anything obvious besides just adding another stroke.

For proper use create a new (fill) material and change the alpha of the fill color to 1.0 or any other value greater than 0. You may want to edit the stroke settings, too, i.e. deactivate it for the dedicated fill material.